Facile solvothermal synthesis of 3D flowerlike β-In₂S₃ microspheres and their photocatalytic activity performance

Abstract

Three-dimension (3D) flowerlike β-In₂S₃ microspheres have been successfully synthesized by a facile solvothermal method using thioacetamide (TAA, CH₃CSNH₂) as both a sulfur source and ligand of In³⁺ in the ethanol–water system. The morphologies of In₂S₃ can be controlled by simply changing the volume ratio of ethanol to water in the solvent. The experimental results demonstrate that 3D flowerlike β-In₂S₃ microspheres undergo surface recrystallization, selective absorption and oriented growth processes. A detailed morphology formation mechanism has been proposed and discussed. Furthermore, the 3D flowerlike β-In₂S₃ microspheres show relatively high visible-light photocatalytic activity for methyl orange (MO) degradation, which can be attributed to both the relative higher BET surface area and advantageous optical properties.

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Introduction

In the quest for environmental purification, photocatalysis using semiconductors and light energy has attracted tremendous attention. The most extensively studied photocatalyst, TiO₂,¹ possesses a wide band gap of 3.0–3.2 eV, which is only active under irradiation of ultraviolet (UV) light accounting for ∼4% of the total sunlight.² To effectively harvest the visible light that is the most abundant in the total sunlight, much work has been done to develop “second-generation” TiO₂ by dye sensitizing and doping,^3–8 which unfortunately can either not show an ideal absorption in the visible-light region or be unstable during the photocatalysis process. In recent years, more and more attention has been paid to developing new visible-light-active photocatalysts (e.g., Ag₃PO₄,⁹ Ag₃VO₄,¹⁰ BiVO₄,¹¹ CdS/graphene,¹² ZnInS₂ (ref. 13). However, these photocatalysts are unstable upon illumination with light (e.g., Ag₃PO₄, Ag₃VO₄), exhibit low activity (e.g., BiVO₄), include highly toxic element (e.g., CdS) or difficult to control synthesis (e.g., ZnInS₂). Compared with above mentioned photocatalysts, the cost of indium sulfide (In₂S₃) is relatively high, because indium is a kind of dispersed element. However, In₂S₃ has tremendous advantages, due to its excellent illumination stability,¹⁴ superior visible light activity,^15–17 stable chemical and physical characteristics and low toxicity.^17,18 At atmospheric pressure, In₂S₃ is found to crystallize into three different structural forms, defective cubic structure (α-In₂S₃), defective spinel structure (β-In₂S₃), and layered hexagonal structure (γ-In₂S₃).^19–30 β-In₂S₃ is the stable state at room temperature with a tetragonal structure or cubic form, which is an n-type semiconductor with a suitable band gap of 1.9–2.3 eV corresponding to visible light region.³¹

It is well-known that the size and morphology of nanomaterials have an important influence on their properties, such as the movement of electrons and holes and the transportation related to phonons.^32,33 Especially micrometer 3D architecture with nano-scale building blocks is effective during the photocatalysis application, considering the enhanced light-harvesting capacity,³⁴ the prevention of aggregation and the easy solid/liquid separation. Morphologically distinct 3D nanocrystals of In₂S₃ including 3D chrysanthemum-like superstructures,^35,36 half shells,³⁷ micropompons,³⁸ hollow microspheres,^16,39 porous 3D flowerlike structures^21,40 have been prepared by various methods. However, among these methods some depend on poisonous organic solvents as the reaction mediates, some demand the surfactants as templates and some need complex or highly toxic sulfur source to control ions activity. Therefore, how to obtain the desired 3D micrometer architectures of In₂S₃ using simple method is still in need of further exploration.

Herein, we put forward a facile solvothermal strategy to synthesize 3D flowerlike β-In₂S₃ hierarchical superstructure assembled by two-dimension (2D) In₂S₃ nanosheet building blocks. In this method, TAA acts both as sulfur source and ligand of In³⁺, which plays an important role in the phase and shape formation of In₂S₃. Compared with the aforementioned methods, simplicity and safety make our pathway dominant (using TAA as sulfur supplier which is a common and harmfulness sulfur source, applying nontoxic ethanol–water mixed solvent as reaction system and without any surfactants). Moreover, morphologies of the In₂S₃ can be controlled by simply changing the volume ratio of water and ethanol. The possible growth mechanism of 3D flowerlike In₂S₃ microspheres is proposed and discussed. Further physical investigations reveal that the unique hierarchical superstructure greatly depends upon the synthesis condition. In addition, the prepared 3D flowerlike In₂S₃ microspheres are applied in photocatalytic degradation of MO to investigate its photocatalytic properties, which show relatively high visible-light photocatalytic activity.

Experimental procedure

Synthesis of 3D flowerlike In₂S₃ microsphere

In a typical procedure, InCl₃·4H₂O (0.5 mmol) and TAA (3 mmol) were dissolved in a mixed solvent of distilled water (10 mL) and absolute ethanol (10 mL) under constant vigorous stirring to yield a homogeneous solution. Then, the solution was transferred into a 33 mL capacity Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 °C for 24 h, and then allowed to cool to room temperature. The resulting powders were filtered and washed with distilled water and absolute ethanol several times to remove the by-products, and finally dried at 60 °C for 4 h under vacuum.

Characterization

The phase compositions of the products were characterized by X-ray diffraction (XRD) on a Rigaku D/max 2500V/PC X-ray diffractometer with Cu-Kα radiation (λ = 1.54056 Å). The morphologies of the products were examined with a JSM 6700F scanning electron microscope (SEM). The element analysis was conducted with an energy-dispersive spectrometer (EDS), an accessory of SEM (JSM 6700F). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) micrographs were taken with a Tecnai G2 F20 transmission electron microscope. The Brunauer–Emmett–Teller (BET) specific surface area was measured by using the nitrogen adsorption–desorption isotherms (BELSORP-Mini) at 77 K. A Shimadzu UV-3600 UV-vis spectrophotometer was used to record the UV-vis diffuse reflection spectroscopy (DRS) of the samples. The photoluminescence (PL) spectra of the products were obtained by a fluorescence lifetime and steady state spectrometer (FSP 920) with an excitation wavelength of 322 nm.

Photocatalytic reactions

The photocatalytic activities of the obtained In₂S₃ products were evaluated by the photocatalytic degradation of MO aqueous solution at room temperature under visible light irradiation. A 250 W Xe lamp with a cutoff filter for UV light and a water cutoff filter for infrared light were used as a light source to provide the visible light. In the test, 0.05 g of as-prepared In₂S₃ dispersed into 100 mL of MO aqueous solution (10 ppm). Before light irradiating, the suspension stirred for 30 min in the dark to reach an adsorption–desorption equilibrium between the photocatalyst and MO solution. At given time intervals, 5 mL aliquots were extracted and centrifuged to remove the In₂S₃ nanocrystals. The filtrates were analyzed by recording variations of the maximum absorption band (λ = 464 nm) in the UV-vis spectra of MO using UV-vis spectrophotometer.

Results and discussion

The phase composition of the In₂S₃ nanocrystals obtained in ethanol–water system with the volume ratio of 1 : 1 for 24 h is investigated by XRD. As shown in Fig. 1a, all these peaks are indexed to the diffraction pattern of cubic β-In₂S₃, meanwhile, the structural parameter of a = 10.774 Å agrees very well with the reported values (JCPDS Card no. 65-0459). Comparing with the standard XRD pattern, the intensity of the (222) peak is strengthened, and, in order to clearly observe the changes of (222), the fitting curve of XRD pattern from 26° to 29.5° is presented in Fig. S1.† The intensity ratio of I₍₂₂₂₎/I₍₃₁₁₎ increases from 0.099 to 0.540 corresponding to standard value and experimental value, respectively, implying the (222) facets have been preferentially exposed.

Fig. 1 (a) XRD, (b) EDS, (c) SEM and (d) high-magnification SEM of 3D flowerlike In₂S₃ microspheres obtained in ethanol–water system with the volume ratio of 1 : 1 for 24 h.

No characteristic peak is observed for other impurities such as InS, In, and S, indicating that pure cubic β-In₂S₃ is formed via the solvothermal process. An EDX spectrum (Fig. 1b) of the products is recorded to analyze chemical compositions of In₂S₃ nanocrystals. The result suggests that uniform 3D flowerlike In₂S₃ microspheres only consist of indium and sulfur with an atomic ratio of 60.19 : 39.81, which is almost consistent with the stoichiometry of In₂S₃. The morphologies of In₂S₃ nanocrystals are studied by SEM. As shown in the low-magnification SEM image (Fig. 1c), the obtained product is composed by many uniform 3D flowerlike microspheres with diameter about 1 μm presenting with imperfect dispersibility. It can be clearly revealed in high-magnification SEM image (Fig. 1d) that the exterior surface of In₂S₃ microsphere exhibits an extensive growth of sheet-like structures. The thickness of these nanosheets mostly is about 10 nm, and these nanosheets align together at different orientation, forming open cavities distributed on the entire surface of the microspheres. The cavities are a few tens of nanometers in width and a few tens of nanometers in deep with the pinnacles pointing to the center of the microspheres. It is expected that such cavities may lead to cavity-mirror effects and large specific surface area.

TEM characterization may provide additional information regarding the interior structure of these architectures. A typical TEM image is shown in Fig. 2a, the dark center of the microsphere suggests that the microsphere has a solid central part while the pale area along the perimeter indicates that the perimeter of the microsphere is built by very thin sheets, which is consistent with the SEM images. A HRTEM image taken at the fringe of In₂S₃ nanosheet is displayed in Fig. 2b, which presents crystalline structural information on the In₂S₃ nanosheet. These nanosheets show a highly single-crystalline, determined by the large-scale clear and regular lattice fringes.

Fig. 2 (a) TEM, (b) HRTEM, (c) enlarged HRTEM images, (inset) corresponding FFT pattern of 3D flowerlike In₂S₃ microspheres obtained in ethanol–water system with the volume ratio of 1 : 1 for 24 h, and (d) crystal structure of cubic In₂S₃ viewed along the [111] direction (S and In).

For better visibility of the lattice fringes, enlarged HRTEM image taken from the square areas has been shown in Fig. 2c. The typical lattice fringe spacing is certified to be 0.38 nm, corresponding to the {220} crystallographic plane families of cubic β-In₂S₃, indicating the preferential growth along the <220> crystallographic direction families. The corresponding hexagonal symmetrical fast Fourier transform (FFT) pattern (inset) further demonstrates that the obtained In₂S₃ nanosheet is in single crystalline structure with preferential growth along the <220> crystallographic direction families. Moreover, the quasi-hexagonal-shaped FFT pattern is a typical feature of the reciprocal lattice projected along the [111] zone axis of cubic β-In₂S₃,^15,27,41 which confirms that the exposed facet is (222) plane, consistent with the XRD result. A typical atomic model obtained from ICSD-202353 is shown in Fig. 2d, further revealing its hexagonal symmetrical structure viewed along the [111] direction.

In order to disclose the formation mechanism of 3D flowerlike In₂S₃ microspheres, the whole synthesis process has been carefully observed and analyzed, including the dissolution process of reactants at room temperature and the reaction process in the high temperature oven. At first, when InCl₃·4H₂O and excess TAA are added to the mixed solvent of ethanol (10 mL) and water (10 mL), uniform and transparent solution can be obtained immediately. Then, the solution is transferred into Teflon-lined stainless steel autoclave and kept for several hours in oven. The products obtained at different reaction durations in the autoclave is collected and analyzed through XRD (Fig. S2†) and SEM (Fig. 3). It could be found that all of the XRD peaks can be readily indexed to cubic β-In₂S₃ phase (JCPDS no. 65-0459) and the intensity of the diffraction peaks has hardly changed, however, the intensity of the (222) peak begins to increase from 2 h and gradually enhances with the reaction time prolonging. This observation implies that the crystal growth of In₂S₃ gets down to adopting oriented growth along <220> crystallographic direction families to expose (222) facet from 2 h. The corresponding SEM images further evidence the shape evolution process of 3D flowerlike In₂S₃ microspheres.

Fig. 3 SEM images of In₂S₃ nanocrystals synthesized at different reaction durations in ethanol–water system with the volume ratio of 1 : 1: (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 8 h, (f) 16 h.

From Fig. 3a–f, we could find that small In₂S₃ nanoparticles about 10 nm in diameter are the exclusive products for 0.5 h (Fig. 3a), and then large microspheres with several hundred nanometers in diameter gradually appear when the reaction time is prolonged to 2 h (Fig. 3b and c). After 2 h (Fig. 3d), a great number of small nanosheets begin to grow on the surface of the microspheres resulting in the preferential exposure of (222) facet. As time going on (Fig. 3e and f), the nanosheets continually grow up and the diameters of these microspheres develop from several hundred nanometers to about 1 μm.

On the basis of the dissolution process of reactants and the above experimental results, we deduce the formation mechanism of 3D flowerlike In₂S₃ microspheres, pictorially shown in Fig. 4. The fast dissolution process of reactants indicates the excess TAA can coordinate with In³⁺ completely and form indium–TAA complexes rapidly, which greatly promote the dissolution of InCl₃·4H₂O and avoid hydrolysis and precipitation of In³⁺.^13,42,43 This ensures a homogeneous environment during the reaction procedure, and makes the reaction carry out in a very low rate to obtain nanocrystals with perfect crystal structure and morphology. Indeed, In³⁺ can form two different complexes with TAA as the tetrahedral [In(TAA)₄]³⁺ and the octahedral [In(TAA)₆]³⁺.^13,43 During the reaction process, along with the rise of temperature and pressure, S²⁻ ions are slowly released by TAA when the violent shaking S–C bond of TAA is broken under the powerfully nucleophilic attack of high-energy water molecules, and as a result, In–S₆ and In–S₄ species are formed.^13,16,43 Then, these freshly generated metal sulfur species combine in situ to reduce their charge density, resulting in a thermodynamically stable cubic phase In₂S₃ nuclei in which the coordination pattern of the ions in solid retain the same style as in the solution. In other words, the coordination manner of the ions in the solution can determine the phase of the solid product.⁴³

Fig. 4 The proposed growth mechanism of 3D flowerlike In₂S₃ microspheres.

At this initial stage, a large number of In₂S₃ nuclei can form in a short time due to the excess S²⁻, resulting in a high degree supersaturation of In₂S₃. Large amounts of small In₂S₃ nanoparticles are further obtained with the continuous supply of the building blocks. At the same time, surplus S²⁻ tends to attach to the surface of products for the electrostatic interaction between S²⁻ and In³⁺ on the surface. These particles in the solution can aggregate to form large microspheres, driven by minimizing the surface energy and hydrogen-bond interaction.^16,44,45 There are large amounts of hydrogen bonds in ethanol–water system which can supply enough interaction force to involve in the process. The plentiful sulfur ions surrounding the microspheres can recombine with solid In₂S₃ to generate liquid In–S₆ or In–S₄ species. Therefore, as time going on, the In₂S₃ on the surface of microsphere dissolve and recrystallize to form sheetlike In₂S₃ nuclei which is bounded by alternant {110} and {222} facets.¹⁵ This dissolution and recrystallization process on the microsphere surface is decisive for the formation of sheetlike structure. It is worth noting that In₂S₃ molecules generated rate at this stage slows down due to the low concentration of reactants. As a consequence, the concentration of In₂S₃ molecules is not enough for the former microspheres to grow from the circumference. The new generated building molecule blocks will preferentially occur at the sheetlike In₂S₃ nuclei which are active sites with higher free energies.⁴⁶ Generally, the final crystal shape is the cooperative result of internal crystal factors and external reaction factors.⁴⁷ According to Gibbs–Wulff's theorem, higher surface tension faces tend to grow along its normal direction and eventually disappear from the final appearance, and a sequence of γ_{222} < γ_{110} can be easily obtained for the cubic phase in light of this theorem.¹⁵ Moreover, in our experiment, TAA might further cap on the (222) plane of In₂S₃ and increase the difference of surface energy between (222) and (110) planes. As shown in Fig. S3,† the (222) plane of cubic In₂S₃ is completely composed by In³⁺, which is facile for the TAA to adsorb to the surface of (222) plane and coordinate with In³⁺. Consequently, the growth of the In₂S₃ crystal along [111] direction is suppressed and In₂S₃ crystal growth mainly processes along the six symmetric directions to form sheetlike In₂S₃. Finally, a flowerlike structure is formed by selective absorption and oriented growth on active sites.⁴⁸ Besides, it may be due to the presence of large amounts of hydrogen bond in ethanol–water system that the 3D flowerlike In₂S₃ microspheres aggregate in a certain degree during the formation process. Based on the above results, the possible overall chemical reactions that occurred during the formation of In₂S₃ could be proposed as shown in eqn (1)–(3):⁴⁹

nTAA + In³⁺ → [In(TAA)₄]³⁺ or (n = 4 or 6) (1)

[In(TAA)₄]³⁺ or [In(TAA)₆]³⁺ + 10H₂O → (InS₄)⁵⁻ or (InS₆)⁹⁻ + 10CH₃CONH₂ + 20H⁺ (2)

(InS₄)⁵⁻ + (InS₆)⁹⁻ ⇌ In₂S₃ + 7S²⁻ (3)

In general, the reaction environment has a great effect on the morphology of the final products. Our experimental results indicate that the solvents indeed play an important role in the formation of products. As can be observed in Fig. S4,† all of the XRD peaks can be readily indexed to cubic β-In₂S₃ phase (JCPDS no. 65-0459), however, the intensity of the (222) peak gradually increases with the water dosage improving. It indicates that increasing the water dosage is conducive for the formation of sheetlike structure. SEM images in Fig. 5 further verify the effect of water on the formation of nanosheets. Nanoparticles and out-of-shape microcrystals composed by nanoparticles with compact structure (Fig. 5a and b) are obtained in pure ethanol or ethanol–water system with the volume ratio of 3 : 1, respectively. Adjusting the volume ratio of ethanol and water into 1 : 3, aggregate structures of flowerlike microspheres are obtained. On the surface of these structures, large amounts of interlinked nanosheets form plenty of open cavities (Fig. 5c). In pure water, numerous microcrystals composed by interlaced In₂S₃ nanosheets aggregate and weld together, in which numerous well-developed open cavities are also observed (Fig. 5d). Based on the decomposition equation of TAA, increasing the amount of water molecules can promote the decomposition reaction and generate more S²⁻. Thus, in this system with more water, there will be more S²⁻ surrounding the products, which greatly facilitate the In₂S₃ dissolution and recrystallization to form sheetlike nuclei on the surface of products and is conducive for the formation of sheetlike structure. Moreover, it is obvious that aggregation of nanocrystals is also attributed to the adding of the water. It could be owing to the more abundant hydrogen bond network in the solvent.⁵⁰ According to the above discussion and experimental results, morphologies of the In₂S₃ can be controlled by simply changing the volume ratio of water and ethanol in solvent. The best volume ratio of ethanol and water to synthesize desired product is 1 : 1, which can make the products possess both sheetlike loose structure and better dispersibility.

Fig. 5 SEM images of products obtained in ethanol–water system with the volume ratio of: (a) 1 : 0, (b) 3 : 1, (c) 1 : 3, (d) 0 : 1.

The specific surface area is an important property of a catalyst to determine its activity to apply for photocatalytic degradation. Large surface areas can effectively reduce the e⁻/h⁺ recombination rate, which will lead to more efficient reaction with the oxidant to produce radicals for dye degradation or directly reaction with the dyes.^51,52 The BET specific surface areas of obtained samples in different solvents are calculated from the nitrogen adsorption–desorption isotherms. As can be seen from Fig. 6a, the isotherms are characteristic of type IV isotherms with a hysteresis loop. 3D flowerlike In₂S₃ powder possesses the largest BET surface area of 72.9 m² g⁻¹, which may be attributed to its loose flowerlike structure and better dispersibility. As speculated, In₂S₃ samples with other compact or aggregate shapes have smaller specific surface areas. The specific surface areas of nanoparticles and microcrystals composed by nanoparticles with compact structure are just 10.4 m² g⁻¹ and 18.3 m² g⁻¹, which are obtained in pure ethanol or ethanol–water system (3 : 1), respectively. Aggregate structures of flowerlike microspheres synthesized in ethanol–water system (1 : 3) and pure water possess surface area of 33.7 m² g⁻¹ and 38.1 m² g⁻¹, respectively. In addition to the structural properties, the optical properties of the In₂S₃ samples are also studied by UV-vis DRS and PL spectra. Fig. 6b shows the typical UV-vis diffuse reflectance spectra of the as-synthesized In₂S₃ nanocrystals. The wide and intense absorption peaks in ultraviolet and visible region reveal strong absorption of ultraviolet light and visible light. Comparing with other products, In₂S₃ composed by nanosheets with more cavities show stronger absorption which are obtained in ethanol–water system (1 : 1, 1 : 3) and pure water. Because, open cavities are conducive to their excellent cavity-mirror effect, leading to a great improvement of the reflection and absorption ability for laser.⁵³ Photons inside the cavities have little chance to escape as they will be reflected many times until most of them are finally absorbed. The calculated pore volume using Barrett–Joyner–Halenda (BJH) method (Table 1) further indicates that the products composed by nanosheets possess more pores. According to the pore size distribution plots (inset of Fig. 6a) of samples, the pore size is not uniform ranging from a few nanometers to a few tens nanometers, which is consistent with the TEM observation. For an indirect gap semiconductor, it is well-known that the relation between absorption coefficient and band gap energy can be described by the formula (αhν)^1/2 = A(hν − E_g) (where α, hν, and E_g are the absorption coefficient, the discrete photon energy, and the band gap energy, respectively; A is a constant).¹⁴ A classical extrapolation approach is employed to estimate the E_g of In₂S₃ products. The extrapolated value (the straight line to the X axis) of hν at α = 0 give the band gap E_g of approximate 1.9 eV (inset of Fig. 6b) which agrees with the experimental reports.⁵⁴ These samples have almost no difference in the band gap energy. It may be due to the size of their building blocks is similar and is not enough to cause quantum confinement effect.⁵⁵ The specific surface area, pore volume, pore size distribution and E_g of different samples are summarized in Table 1.

Fig. 6 (a) Nitrogen adsorption–desorption isotherm (inset: BJH pore size distribution plot); (b) UV-vis DRS spectra (inset: plots of (αhν)^1/2 vs. hν); (c) room temperature PL spectra of as-synthesized products in different solvents and (d) photocatalytic degradation curve of MO (10 ppm) degradation applying In₂S₃ obtained in different solvents (blank: with visible light irradiation but without any photocatalysts).

Table 1 Measured parameters for the samples synthesized in different solvents

Sample	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)	Eg (eV)
Ethanol	10.412	0.0489	18.783	1.91
Vethanol : Vwater 3 : 1	18.324	0.0526	11.472	1.90
Vethanol : Vwater 1 : 1	72.928	0.1149	12.076	1.93
Vethanol : Vwater 1 : 3	33.692	0.0774	9.1960	1.89
Water	38.074	0.1104	6.0572	1.92

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PL spectra originate from the migration, transfer, and separation efficiency of the photogenerated charge carriers in a semiconducting material. There is a strong correlation between PL intensity and the photocatalytic performance. Higher PL intensity indicates the higher recombination of the charge carriers, which results in lower photocatalytic activity. The comparison of PL spectra of the as-synthesized In₂S₃ nanocrystals for excitation wavelength 322 nm is shown in Fig. 6c. The two emission peak centers at 467 nm and 472 nm can be attributed to the presence of several deep trap states or defects in the structure.^25,37,40 As we mentioned above, large specific surface area indeed effectively reduce the recombination rate of the charge carriers. Comparing Fig. 6c with Fig. 6a, it can be discovered that the order of PL intensity is on contrary to the specific surface area of products. And 3D flowerlike In₂S₃ products which have the largest specific area possess the minimum relative intensity of PL spectra. Consequently, the advantageous optical property and large specific surface area may endow the as-prepared 3D flowerlike In₂S₃ microspheres with the best potential applications of effective photocatalysis under visible-irradiation in our experiment.

The visible-light photocatalytic activity of 3D flowerlike In₂S₃ microspheres, and some comparative experiments were evaluated by degradation of MO aqueous solution. Under visible-light irradiation, the photocatalytic results of MO are showed in Fig. S5† and 6d. Temporal changes in the concentration of MO as monitored by the maximal absorption in UV-vis spectra at 464 nm over the 3D flowerlike In₂S₃ microspheres are showed in Fig. S5.† It is found that the intensity of the absorption peak of MO is decreased gradually with the irradiation time increasing, accompanied with the color change from the initial orange to nearly colorless. Meanwhile, the main absorption peak position has almost no change and no other absorption band appears, which suggests that the whole process is dominated by photocatalytic degradation mode other than dye sensitization. Fig. 6d shows the variation in absorption of MO at 464 nm with the passage of irradiation time. The y-axis of degradation is reported as C/C₀, C is the absorption of MO at each irradiated time interval of main peak of absorption spectrum at wavelength 464 nm and C₀ is the absorption of starting concentration. As shown in Fig. 6d, left of dotted line, the absorption–desorption equilibriums are established after 30 min in dark absorption of In₂S₃ photocatalysts, so the light was turned on after 60 min of dark absorption. The blank experiment without In₂S₃ photocatalyst indicates that direct photocatalysis of MO under the same conditions can almost be neglected. After visible light irradiation 30 min, MO is degraded by 42.7% over as-prepared 3D flowerlike In₂S₃ microspheres and 93.1% for 180 min, where the reaction time is deducted the time of adsorption equilibrium. However, there is almost no degradation as the solution with In₂S₃ nanoparticles which are obtained in pure ethanol. Moreover, after visible light irradiation 180 min, MO is just degraded by 40% over In₂S₃ sample collected in ethanol–water system (3 : 1), 66% for product in ethanol–water system (1 : 3) and 70% for the sample obtained in pure water. These results reveal that the as-synthesized 3D flowerlike In₂S₃ microspheres have the best photocatalytic activity for MO, which can be attributed to both the relative higher BET surface area and advantageous optical property. As to the photocatalytic mechanism of In₂S₃ on MO, Fu et al. verified that visible light excited In₂S₃ leads to formation of ·OH radical and in turn oxidizes the organic compound pollutants by the generated ·OH radical.^14,56

e⁻ + O₂ → ·O²⁻ (4)

·O²⁻ + e⁻ + H⁺ → H₂O₂ (5)

·O²⁻ + H₂O₂ → ·OH + OH⁻ + O₂ (6)

H₂O₂ → 2·OH (7)

MO + ·OH → CO₂ + H₂O + NH₄⁺ + SO₃²⁻(SO₄²⁻) (8)

Finally, MO can be oxidized and mineralized to produce inorganic compounds, such as CO₂, H₂O, NH₄⁺, SO₃²⁻, and SO₄²⁻. The possible overall reactions that occurred during this photocatalysis process could be proposed as shown in eqn (4)–(8):

Conclusions

In summary, 3D flowerlike β-In₂S₃ microspheres have been successfully synthesized by a facile thermal solution method only using ethanol–water as solvent. In this work, TAA not only acts as a sulfur source but also a ligand of In³⁺, which plays an important part in the phase formation and the morphologies evolution of In₂S₃. The reaction durations and solvent as well have a significant influence on the morphology of the In₂S₃ products. A morphology formation mechanism has been proposed and discussed on the basis of experimental data. Furthermore, the prepared 3D flowerlike In₂S₃ microspheres are applied in photocatalytic degradation of methyl orange and show relatively high visible-light photocatalytic activity, which can be attributed to both the large BET surface area and advantageous optical property. Our method is a simple and safe route that involves no complex or highly toxic reagents, surfactants or poisonous organic solvents. Therefore, it is very promising for simple, safe and low-cost industrial production.

Acknowledgements

This work was supported by the National Natural of Science Foundation of China (Grant no. 21371101), 111 Project (B12015) and MOE Innovation Team (IRT13022) of China.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1: fitting curve (26° to 29.5° in 2θ) of 3D flowerlike In₂S₃. Fig. S2: XRD patterns of samples synthesized at different reaction durations in ethanol–water system (1 : 1). Fig. S3: schematic diagram of a projected view as TAA absorbed on the (222) surface of In₂S₃ to form a layer. Fig. S4: XRD patterns of products obtained in ethanol–water system with the volume ratio of 1 : 0, 3 : 1, 1 : 3, 0 : 1. Fig. S5: time-dependent UV-vis absorption spectra using 3D flowerlike In₂S₃ as photocatalyst. See DOI: 10.1039/c4ra08545k

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